1. Field of the Invention
The present invention relates to a semiconductor light emitting device (e.g., a semiconductor laser device) capable of emitting light in a wavelength range of blue to ultraviolet light, and a method for fabricating such a semiconductor light emitting device. More particularly, the present invention relates to a current-blocking type gallium nitride (GaN) based compound semiconductor light emitting device (e.g., a semiconductor laser device) with high reliability, and a method for fabricating such a semiconductor light emitting device.
2. Description of the Related Art
Conventionally, a gallium nitride based compound semiconductor laser device is formed on a crystal by lateral growth using an insulating film such as an SiO.sub.2 film as a selective growth mask. FIG. 11 is a cross-sectional view of a conventional semiconductor laser device fabricated by such a conventional technique.
The conventional semiconductor laser device of FIG. 11 includes a sapphire substrate 1100, a GaN underlying layer 1101, a stripe-shaped insulating selective growth mask 1102 made of SiO.sub.2, an n-type GaN contact layer 1103, an n-type AlGaN cladding layer 1104, an n-type GaN optical guide layer 1105, an In.sub.0.2 Ga.sub.0.8 N/In.sub.0.05 Ga.sub.0.95 N multi-quantum well active layer 1106, a p-type GaN optical guide layer 1107, a p-type AlGaN cladding layer 1108 having a ridge stripe structure 1120 located above the SiO.sub.2 selective growth mask 1102, a p-type GaN contact layer 1109, an n-type electrode 1110 formed on the n-type GaN contact layer 1103, and a stripe-shaped p-type electrode 1111 formed on the p-type GaN contact layer 1109.
The n-type GaN contact layer 1103 is continuously formed on the insulating selective growth mask 1102. Actually, crystal growth of GaN for the n-type GaN contact layer 1103 starts on the portions of the GaN underlying layer 1101 corresponding to the openings of the insulating selective growth mask 1102, i.e., the portions which are not covered with the insulating selective growth mask 1102. As the growth proceeds in the thickness direction, the GaN layer gradually extends over the respective stripes of the insulating selective growth mask 1102 from both sides of the stripes, finally covering the insulating selective growth mask 1102. Thus, the n-type GaN contact layer 1103 is formed as a single layer.
In the above conventional semiconductor laser device, a current injected from the n-type electrode 1110 flows in the n-type GaN contact layer 1103 in a lateral direction, and electrons in the current recombine with holes existing in the area of the active layer 1106 located right under the ridge stripe structure 1120, thereby generating light.
In the conventional semiconductor laser device with the above configuration, electrons inevitably travel in the n-type GaN contact layer 1103 in the lateral direction to reach the p-type contact layer 1109. More specifically, electrons are required to cross the areas of the n-type GaN contact layer 1103 located above the stripes of the insulating selective growth mask 1102.
However, according to the above-mentioned conventional process, minute crystal cracks extending in the direction of crystal growth and non-grown portions tend to be formed in areas of the n-type GaN contact layer 1103, indicated by reference numeral 1150, which correspond to the centers of the respective stripes of the insulating selective growth mask 1102. This tends to block smooth flow of electrons in the n-type GaN contact layer 1103 in the lateral direction.
As a result, the series resistance of the above conventional semiconductor laser device is as high as about 45 to 140 .OMEGA.. Such a high series resistance causes heat generation and crystal distortion, which in turn reduce the lifetime of the device to 150 hours or shorter under the conditions of an ambient temperature of 60.degree. C. and a light output of 5 mW. Such a short lifetime is not suitable for application to optical disk systems and the like.
In addition, electric field tends to concentrate in the cracks and the non-grown portions. This increases the operating voltage of the semiconductor laser device to about 15 to 30 V and often causes breaking of semiconductor laser devices.